This has been a bug for a while, it has been happening ever since they introduced variable number of realms based on how busy the server is.
What you describe happens when the server wants to reduce the number of realms.
Example:
EUE server has 4 portals, lets say,: Medusa, Beholder, Sprite, Imp.
EUE Beholder closes.
The server does some calculations (no idea how) and decides that Beholder is not going to reopen because there aren’t enough players to get 4 realms.
So the EUE nexus will remain at Medusa, Sprite, Imp.
Now anyone in a dungeon in closed-Beholder, when they go into an exit portal will go to the nexus instead of returning to the closed realm.
Edit: earlier topic about this bug: Closed Realm Dungeon Leave Nexus
Edit2: this could have got worse because in this update they said they made some changes to how the game assigns names to the realms, so maybe something else got broken.
I haven’t seen it happen for myself today, but I know EUE was on 4 realms earlier today, (and I remember your name from being on EUE) so guessing it could be this.
It has been happening a bit recently on EUW too, when the server decreases from 4 realms.
Or EUE could be a special case/there could be something else going on, I did see what looked like about 20 bots on EUE trying to connect and reconnect earlier on there today, flashing appearing/disappearing in the nexus spawn. Maybe they were screwing with the server’s player count/busy algorithms and you suffered the side-effects.